Electric discharge tube comprising a secondary emission electrode



Aug. 2, 1949. A. J. w. M. VAN OVERBEEK EI AL 2,478,141

' ELECTRIC DISCHARGE TUBE COMPRISING A SECONDARY EMISSION ELECTRODE Flled May 7, 1946 M w T a li 605m.

ATTO R N EY Patented Aug. 2, 1949 RIC ,D QH R TU E HQQMBRISI A SECONDARY EMISSION ELECTRODE- Adrianus Johannes Wilhelm'us Marie van- Over: beck, Gerrit Hendrik Petrus Alma; and Gare! Peter Klopping, E-indhoven, Netherlands, assignors, by mesne assignments, to Hartford National Bank and Trust company, Hartford,

Conn., as trustee Application May 7, 1946,: Serial No. 667,832 In the Netherlands-May 16,1942

Section 1,- Publi'c'Law 690; August 8', i946" Patent expires May 16, 1962 2 Claims;

This invention relates to an electric discharge tube comprising an electrode system having at least one secondary emission electrode which is coated with a material having a high capacity for secondary emission and consisting of com-' pounds of alkali metal. Such -secondaryemission electrodes are also called auxiliary cathode.

It is known that the number of secondary electrons which an electrode coated with alkali compounds containing a small quantity of alkali metal is capable of emitting per impinging electron depends on the quantity of free metal. With a definite quantity of free metal the maximum secondary-emission factor occurs which is to be understood to mean the number indicating how many secondary electrons per impinging primary electron are dislodged.

It has turned out that if the vapour pressure of the alkali metal in the tube is kept constant the quantity of free metal on the secondary-emission elecrode mainly depends on the temperature of the secondary-emission electrode which in its turn mainly depends on the load thereof. With a high load that is with a large number of impinging primary electrons the secondary-emission electrode will acquire a higher temperature as a result of which a quantity of alkali metal will evaporate away. With a small load the auxiliary cathode acquires a lower temperature so that there will be a larger quantity of alkali metal on the electrode. At a definite temperature of the auxiliary cathode just that quantity of alkali metal will be available which yields the maximum secondary-emission factor. When making use of a getter combining with alkali metal to such a degree that its vapour pressure in the tube becomes substantially equal to zero the load of the auxiliary cathode can consequently not be chosen higher than a definite value, since in this case the temperature of these electrodes rises to a value at which the secondary emission factor can no longer have its maximum value. In fact, alkali metal evaporates from the electrode and is directly bound. In the tubes hitherto known this is a serious drawback of the use of secondary-emission electrodes whose surface consists of an active layer containing alkali metal compounds and a small quantity of alkali metal. However this temperature can be controlled by artificial cooling or by providing coatings having a high-radiating capacity. This, however, renders the construction of the electrode system more complicated, whilst one must always remain below a definite load of the aux- (Cl. 525M174) 2. iliar'y cathode. Now, the invention provides means for avoiding these drawbacks.

An electric discharge tube according to the invention has an electrode system comprising the usual electrodes such as cathode, control-electrode, anode or the like and in addition at least one secondary-emission electrode, which is coated with an active layer consisting of compounds of alkali metals and a small quantity of alkali metal, the characteristic feature being that the tube contains a special adsorption getter capable of combining with alkali metal, the quantity' of this getter being such that the secondaryemission factor of the auxiliary cathode has its maximum value during" operation of the tube with the prescribed voltages and currents.

As regards the semondary-emission electrodes electric discharge tubes designed according to the invention, can be loaded, whilst retaining the maximum secondary-emission factor, to a higher degree than the tubes hitherto known, in which use is made of thesame electrode system, with-- out using either constructional means or technological means, for cooling the auxiliary cath- In the present case a special adsorption getter is to be understood to mean a material which at the temperature occurring in the tube is capable of taking up alkali metal at a definite vapour pressure and of releasingit soon again upon a decrease of the vapour pressure, so that the vapour pressure is kept constant within definite limits. This includes, for instance, ad sorption means such as active carbon and definite metals, for example, tin and lead.

The explanation of the fact that in tubes according to the invention the secondary-emission factor has its maximum value during operation of the tube is probably as follows: By the special adsorption getter a definite vapour pressure of the alkali metal is maintained in the tube, which pressure depend-s practically solely on the quantity, of getter, provided care be taken that the temperature of the getter remains constant. Consequently, the pressure in the tube is not altered by evaporation of a slight quantity of alkali metal from the auxiliary cathode. The temperature of the auxiliary cathode is determined by its load which is previously determined for each particular kind of tubes in connection with the function which the tube will have to fulill later on, At thistem'perature there is equilibrium between the vapor pressure in the tube which, as hereinbefore stated, is practically constant, and the quality of alkali metal on the auxiliary cathode. Since the quantity of alkali metal determines the secondary-emission factor, the vapour pressure in the tube must be such, when it is desired to give this factor its maximum value, that at the temperature of the auxiliary cathode this quantity of alkali metal is indeed in equilibrium with the prevailing vapour pressure. According to the invention this vapour pressure is given the correct value for complying with this condition by the choice of the quantity of getter. Briefly stated the quantity of getter is consequently so chosen that the vapour pressure of the alkali metal in the tube corresponds to the vapour pressure of the alkali metal prevailing above the secondary-emission electrode at the temperature which the latter acquires during operation of the tube with the prescribed voltages and currents.

One consequence of the vapour pressure being determined by the quantity of getter in such manner that it corresponds to the vapour pressure above the loaded auxiliary cathode is that with a smaller load, at which the temperature falls, the quantity of alkali metal on the auxiliary cathode becomes larger than is admissible for the maximum secondary-emission factor. A higher load of the auxiliary cathode also involves a decrease in secondary emission, since in this case the quantity of alkali metal becomes smaller than corresponds to the maximum secondaryemission factor,

In the manufacture of tubes according to the invention the quantity of getter material is so chosen, as has been stated above, that the secondary-emission factor attains its maximum value at the prescribed voltages and currents, the procedure then being such that a small number of perfectly uniform test tubes is manufactured and the quantity of getter material is made different in each tube. The tubes are then treated in exactly the same manner for degasification, formation of the cathode and so on. By simple determination of the current strength of the auxiliary cathode in testing the tubes at the prescribed voltages it can then be ascertained which tube has the maximum secondary-emission factor. In mass production of tubes of the same kind that quantity of getter material is used which exhibited the maximum secondary-emission factor in the test series.

A tube according to the invention may be built as follows. The auxiliary cathode consisted of a copper strip oxidised in the air. After mounting it in the electrode system, which otherwise was built in a normal manner, the tube was completed by providing a cap containing 110 mgs. of

a mixture of one part caesium chromate and four parts of zirconium metal. From this mixture caesium can be set free by heating. Furthermore a metal cap was introduced into the tube, which had been coated with a thin layer of active carbon obtained by precipitating a quantity of 100 mgs. from a colloidal soluion. In fact, a test series proved that in this particular kind of tube the maximum secondary-emission factor was obtained with this quantity of getter material. After exhausting the tube and forming the primary cathode caesium was set free by heating. The tube was sealed and operated for one hour, followed by further heating for one hour at a temperature of 160? in a furnace without a voltage being impressed on the tube. On testing the tube it appeared that at the prescribed load the 4 secondary-emission factor amounted to 5.5 to 6, whereas in the case of a smaller load it amounted only to 4.5 to 5.

In a particular embodiment of the invention the getter is provided either on or in the vicinity of the secondary-emission electrode so that the temperature of the getter material is determined by the temperature of the secondary-emission electrode. Thus, it is ensured that the vapour pressure in the tube becomes dependent on the temperature of the auxiliary cathode and consequently on its load. In this way it can be provided that such a quantity of alkali metal on the auxiliary cathode is steadily in equilibrium with this vapour pressure which yields the maximum secondary-emission factor at the temperature determined by the load.

According to a particular embodiment of the invention the size, the nature and the location of the auxiliary cathodes in a discharge tube having a plurality of secondary-emission electrodes subjected to different loads may be so chosen that they acquire about the same temperature notwithstanding the different loads. In this case each electrode operates with its maximum secondary-emission factor. Such an adjustment is very useful, for example, in electron multiplier tubes.

We claim:

1. An electron discharge tube comprising an envelope and within said envelope a getter and an electrode structure comprising a cathode electrode, a control electrode, an anode and a secondary emission electrode, said secondary emission electrode being coated with an active layer consisting of an alkali metal compound and a free alkali metal, said getter comprising an alkali combining material in an amount at which the emission factor of said secondary emission electrode is a maximum value under normal operating conditions of said tube.

2. An electron discharge tube comprising an envelope and within said envelope a getter and an electrode structure comprising a cathode electrode, a control electrode, an anode and a secondary emission electrode, said secondary emission electrode being coated with an active layer consisting of an alkali metal compound and a free alkali metal, said getter comprising an alkali combining material in an amount at which the vapor pressure of the alkali metal within said envelope corresponds to the vapor pressure of the alkali metal prevailing above said secondary emission electrode under normal operating conditions of said tube.

ADRIANUS JOHANNES WILHELMUS MARIE VAN OVERBEEK. GERRTI HENDRIK PETRUS ALMA. CAREL PETER KLCSPPING.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

